A variety of applications require the use a uniform line image formed from a high-power laser beam. One such application is laser thermal processing (LTP), also referred to in the art as laser spike annealing (LSA) or just “laser annealing,” which is used in semiconductor manufacturing to activate dopants in select regions of a semiconductor wafer when forming active microcircuit devices such as transistors.
One type of laser annealing uses a scanned line image formed from a laser beam to heat the surface of the semiconductor wafer to a temperature (the “annealing temperature”) for a time long enough to activate the dopants but short enough to minimize dopant diffusion. The time that the surface of semiconductor wafer is at the annealing temperature is determined by the power density of the line image, as well as by the line-image width divided by the velocity at which the line image is scanned (the “scan velocity”).
One type of high-power laser that is used for laser annealing applications is CO2 laser. Traditional methods of performing laser annealing with a CO2 laser including imaging the laser beam onto a pair of knife-edges and then relaying the laser beam passing therethrough to an image plane to form the line image. The knife-edges are positioned to transmit only a narrow central portion (e.g., 10%) of a Gaussian laser beam for which the intensity is relatively uniform so that the resulting line image is also relatively uniform along the length of the line image.
Unfortunately, using only the narrow central portion of the laser beam means that the other 90% of the light beam is rejected. This is a very inefficient use of the high-intensity laser beam. On the other hand, the conventional wisdom is that trying to pass a larger portion of the Gaussian beam will naturally result in non-uniformity of the line image along its length because of the substantial drop off in intensity in the Gaussian beam with distance from the center of the beam.